Thomas Taubner

Dielectric nanoparticles and microparticles show rich optical responses due to their electric and magnetic multipolar Mie modes and low losses. These optical resonances can be modified via the particle size, shape, and dielectric environment. Here we tune the infrared multipolar Mie resonances of dielectric polystyrene microspheres using the plasmonic phase-change material In3SbTe2 (IST). IST exhibits a phase change between a dielectric amorphous phase and a crystalline phase that is metallic in the infrared. We show that a reversibly switchable particle-on-mirror geometry can be created by locally switching the IST under the polystyrene microspheres. Measuring the light-scattering spectra of single polystyrene microspheres before and after crystallization and reamorphization, we observe huge shifts of the resonance wavelength, e.g., from 5.2 to 8.9 μm and back for 6 μm diameter spheres. Our work is a first step toward a controlled tuning of multipolar Mie modes of various particles using reprogrammable IST mirror structures.

Optical metasurfaces composed of metallic or dielectric scatterers (meta‐atoms) promise a powerful way of tailoring light‐matter interactions. Phase‐change materials (PCMs) are prime candidates for non‐volatile resonance tuning of metasurfaces based on a refractive index change. Precise resonance control can be achieved by locally applying laser pulses to crystallize a PCM, modifying the dielectric surrounding of meta‐atoms. However, the complex crystallization kinetics of PCMs in the vicinity of metallic meta‐atoms have not been studied yet. Here, we experimentally investigate metallic dimer antennas on top of the PCM Ge ₃ Sb ₂ Te ₆ and address these nanoantennas with laser pulses. Our study reveals inhomogeneous crystallization caused by the absorption and heat conduction of the metallic nanoantennas. A self‐consistent multiphysics model, including electromagnetic, thermal, and phase‐transition processes, is employed to simulate the crystallization and understand the resulting resonance shift of the antennas. Our work paves the way toward complex antenna geometries optimized for local addressing of PCMs to achieve sophisticated crystallization patterns, enabling on‐demand programming of individual nanoantennas within metasurfaces.

Polaritons in van-der-Waals materials (vdWMs) promise high confinement and multiple tailoring options by resonators and launching structures which conventionally require cumbersome lithography techniques. Optical programming of phase-change materials (PCMs) offers fast and reconfigurable fabrication of these structures. As the plasmonic PCM In₃SbTe₂ can be switched between a dielectric and metallic phase, In₃SbTe₂ is promising for optical programming of metallic launching structures to tailor and confine polaritons in vdWMs. Here, we combine the vdWM hexagonal boron nitride (hBN) with In₃SbTe₂ and optically program circular resonators for hBN's phonon polaritons into In₃SbTe₂. We investigate the polariton resonators with near-field optical microscopy. Demonstrating the reconfigurability, we decrease the resonator diameter to increase the confinement up to λ/39 and achieve a quality factor of 72. Finally, we fabricate a focusing structure for hBN's polaritons whose focal point we reconfigure. We promote In₃SbTe₂ as a versatile platform for rapid prototyping of polariton optics in vdWMs.

Metasurfaces have evolved as prime candidates for miniaturized optical components to tailor the light-matter interaction. Conventionally, the fabrication and design of such metasurfaces with complex antenna structures are well-developed and precise but need clean-room access and multiple fabrication steps. While metasurfaces based on polarization conversion are limited to low efficiencies, metasurfaces with antennas above a metallic mirror reach much higher efficiencies by keeping the initial polarization. Here, we showcase rapid prototyping of reflective beam-steering metasurfaces to optimize their efficiency. We exploit the nonvolatile insulator-to-metal transition of the infrared plasmonic phase-change material In₃SbTe₂ (IST). Via direct laser writing, we optically program reflective beam-steering metasurfaces with grating strips of varied width into thin IST layers and measure the anomalous reflection at a wavelength of 8.6 μm. Reconfiguring the grating bar widths and hence the covered phase range within the supercell leads to improved anomalous reflections of up to 75%. Our work enables reconfigurable, high-efficiency metasurfaces in the infrared range.

Confined electron systems, such as 2D electron gases (2DEGs), 2D materials, or topological insulators, show great technological promise but their susceptibility to defects often results in nanoscale inhomogeneities with unclear origins. Scattering-type scanning near-field optical microscopy (s-SNOM) is useful to investigate buried confined electron systems non-destructively with nanoscale resolution, however, a clear separation of carrier concentration and mobility was so far impossible in s-SNOM. Here, we predict a previously inaccessible characteristic “fingerprint” response of the prototypical LaAlO₃/SrTiO₃ 2DEG, and verify it using a state-of-the-art tunable narrow-band laser in mid-infrared cryo-s-SNOM at 8 K. Our modeling allows us to separate the influence of carrier concentration and mobility on fingerprint spectra and to characterize 2DEG inhomogeneities on the nanoscale. Finally, we model the surface accumulation layer in doped InAs, to show that our fingerprint spectra are a universal feature and generally applicable to confined electron systems, like topological insulators or stacked van-der-Waals materials.

Polaritons in van-der-Waals materials (vdWM) promise high confinement and multiple tailoring options by optical structures, e.g., resonators, launching structures and lenses. These optical structures are conventionally fabricated using cumbersome multi-process lithography techniques. In contrast, phase-change materials (PCMs) offer fast and reconfigurable programming of optical structures. PCMs can reversibly be switched between two stable phases with distinct permittivities by local heating, e.g., by optical laser pulses. While the well-known dielectric PCM GeSbTe-alloys feature only a permittivity change, the PCM In3SbTe2 can be switched between a dielectric and metallic phase. This makes In3SbTe2 promising for programming metallic launching structures. Here, we demonstrate direct optical programming and thereby rapid prototyping of optical launching structures in In3SbTe2 to tailor and confine polaritons in vdWM. We combine the vdWM hexagonal boron nitride (hBN) with In3SbTe2 and optically program circular resonators for hBN's phonon polaritons through hBN into In3SbTe2. We investigate the polariton resonators with near-field optical microscopy. Demonstrating the reconfigurability, we decrease the resonator diameter to increase the polariton confinement. Finally, we fabricate focusing structures for hBN's phonon polaritons whose focal point is changed in a second post-processing step. We promote In3SbTe2 as a versatile platform for rapid prototyping of polariton optics in vdWM.

Conventional optical elements are bulky and limited to specific functionalities, contradicting the increasing demand of miniaturization and multi-functionalities. Optical metasurfaces enable tailoring light-matter interaction at will, especially important for the infrared spectral range which lacks commercially available beam-shaping elements. While the fabrication of those metasurfaces usually requires cumbersome techniques, direct laser writing promises a simple and convenient alternative. Here, we exploit the non-volatile laser-induced insulator-to-metal transition of the plasmonic phase-change material In₃SbTe₂ (IST) for optical programming of large-area metasurfaces for infrared beam-shaping. We tailor the geometric phase of metasurfaces with rotated crystalline IST rod antennas to achieve beam steering, lensing, and beams carrying orbital angular momenta. Finally, we investigate multi-functional and cascaded metasurfaces exploiting enlarged holography, and design a single metasurface creating two different holograms along the optical axis. Our approach facilitates fabrication of large-area metasurfaces within hours, enabling rapid-prototyping of customized infrared meta-optics for sensing, imaging and quantum information.

Metasurfaces have emerged as a versatile tool for tailoring light‐matter interaction. Combining multiple metasurfaces and antenna arrays in a multi‐layer structure promises intrinsic chiral structures as well as circumventing conventional metasurface drawbacks such as chromatic aberrations. For non‐volatile metasurface tuning, phase‐change materials (PCMs) have evolved as prime candidates. Recently, the plasmonic PCM In ₃ SbTe ₂ (IST) has been established which can be reversibly switched from an amorphous dielectric to a crystalline metallic state in the infrared via laser irradiation. While optically reconfiguring antenna shapes with IST has been extensively studied in a single layer structure, fabrication of a multi‐layer IST metasurface for individual resonance control in the different layers has not been shown yet. Here, a proof‐of‐concept of individual resonance control is shown in a dual‐layer IST metasurface with precise laser pulses from the top and from the bottom through the substrate. The complex switching dynamics and temperature distribution with Multiphysics simulations are theoretically explored. Subsequently, two antenna structures in both layers for multiple polarization dependent resonances are investigated. Finally, chiral meta‐atoms are combined as possible applications for dual‐layer IST metasurfaces. A wide range of applications is envisioned for dual‐layer metasurfaces by rapid prototyping chiral and reconfigurable antenna structures.

Encapsulating few‐layer graphene (FLG) in hexagonal boron nitride (hBN) can cause nanoscale inhomogeneities in the FLG, including changes in stacking domains and topographic defects. Due to the diffraction limit, characterizing these inhomogeneities is challenging. Recently, the visualization of stacking domains in encapsulated four‐layer graphene (4LG) has been demonstrated with phonon polariton (PhP)‐assisted near‐field imaging. However, the underlying coupling mechanism and ability to image subdiffractional‐sized inhomogeneities remain unknown. Here, direct replicas and magnified images of subdiffractional‐sized inhomogeneities in hBN‐covered trilayer graphene (TLG) and encapsulated 4LG, enabled by the hyperlensing effect, are retrieved. This hyperlensing effect is mediated by hBN's hyperbolic PhP that couple to the FLG's plasmon polaritons. Using near‐field microscopy, the coupling is identified by determining the polariton dispersion in hBN‐covered TLG to be stacking‐dependent. This work demonstrates super‐resolution and magnified imaging of inhomogeneities, paving the way for the realization of homogeneous encapsulated FLG transport samples to study correlated physics.

Surface plasmon polaritons (SPPs) describe the excitation of photons coupled with free charge carriers at the interface of metals (visible) or doped semiconductors (infrared). While SPPs in the mid-infrared spectral range have been demonstrated in 2D materials such as graphene, their short propagation length combined with weak confinement in bulk materials has prevented real-space imaging of those SPPs. Here, we demonstrate real-space imaging of propagating SPPs on the doped semiconductors CdO and InAs with tunable plasma frequencies in the infrared via scattering-type scanning near-field optical microscopy. Adding a thin film of phase-change materials (PCMs) to these doped semiconductors increases the polariton confinement, leading to simplified SPP imaging and SPP resonator fabrication. We investigate optically written circular resonators of the plasmonic PCM In ₃ SbTe ₂ on CdO with near-field spectroscopy and Fabry-Perot resonators of the dielectric PCM Ge ₃ Sb ₂ Te ₆ on InAs with far-field spectroscopy. Our work enables rapid prototyping of reconfigurable SPP resonators in mid-infrared.